1. Field of Invention
The invention relates to a light actuator and, in particular, to a light actuator that can rapidly warm up a lighting load and keep it illuminating in a stable way.
2. Related Art
The cold cathode fluorescent lamp (CCFL) has wide applications. For example, it is used as the backlit source of liquid crystal display (LCD) screens, in scanners, and in multiple function peripherals (MFP). Other applications such as the transparent media adaptor (TMA), the scan of negative film or XPA also use the CCFL as the light source.
Since the CCFL requires an extremely high voltage (about several hundred volts) when it is started and working, the driving device or inverter has to be able provide such a high-voltage output. The output power quality of the driving device determines the lighting quality and stability of the CCFL.
To ensure the quality of the light source, the temperature of the CCFL has to reach a certain temperature. Therefore, normal scanners have to spend some time to warm up the CCFL after it is turned on. This is why the CCFL of many existing scanners requires a warmup time of about three minutes. In colder areas or environments, the warmup time may be even longer.
Therefore, methods for rapidly warming up the CCFL have been proposed. For example, in the U.S. Pat. No. 5,907,742 uses a dual input voltage control method. That means a higher input voltage (about 12V) is used to make the temperature of the CCFL quickly rise to its work temperature. A lower input voltage (about 8V) is then used after the warmup. Although the method can reduce the CCFL's warmup time down to about 30 seconds, it nevertheless makes the CCFL to sustain a higher current during the warmup time, thereby resulting in a shorter lifetime.
Secondly, to have two different input voltages (for warmup and work times), it uses a pulse width modulation control circuit to control the input voltage. Their circuit design is more complicated.
Moreover, the above-mentioned patent uses the frequency oscillator built in the inverter. The oscillatory frequency often drifts randomly between 35 and 45 kHz under different voltages and temperatures. Consequently, the lighting may become unstable and thus affect the scanning quality.
Another rapid warmup method is to install a NiCr wire coiling around the CCFL. Using the heat generated by the NiCr wire, the lamp tube is forced to raise its temperature. Although this method can achieve the goal of rapidly warming up the CCFL, the addition of the NiCr wire complicates the manufacturing process. Secondly, the NiCr wire also increases the cost, including the equipment and power consumption. The most serious problem is the light blocking by the heating wire around the lamp tube. Therefore, the light intensity of the CCFL is inhomogeneous and affects the scanning quality. Furthermore, the NiCr wire is fragile and thus increases the maintenance cost.
Therefore, current methods of rapidly warming up the CCFL have their own limitations and problems. It is an important subject of the field how to rapidly warm up the CCFL at the same time elongating its lifetime and maintaining a stable light source.